In engine control systems, as proposed in JP 60-240875, an engine rotation speed is detected based on an interval between pulse signals successively generated by a crank angle sensor during engine rotation.
During engine rotation, cylinders are discriminated based on output signals of a crank angle sensor and a cam angle sensor indicative of crankshaft rotation angles (angular positions) and camshaft rotation angles (angular positions). Ignition timing and fuel injection are also controlled based on those output signals. When an engine is started by a starter, it is not clear to which cylinder fuel and ignition should be supplied first until a specified cylinder is discriminated.
It is therefore proposed to store in a memory a crank angle detected by a crank angle sensor when the engine stops as an engine rotation stop position. This stored crank angle is used as a reference to start ignition control and fuel injection control until a predetermined crank angle of a specified cylinder is detected at the time of next engine starting.
As crank angle sensors for engine control systems, an electromagnetic pick-up type sensor is used. This electromagnetic sensor cannot generate large induction voltages at low engine speed conditions, which may occur right after the engine is started or immediately before the engine is stopped. Therefore, low engine rotation speeds or stop position cannot be accurately detected based on the output signal of the electromagnetic sensor.
Further, although the engine sometimes rotates in reverse immediately before its stop due to compression pressure of the engine in the compression stroke, this reverse rotation cannot be detected from the pulse signal of the electromagnetic sensor. As a result, the pulse signal of the electromagnetic sensor may erroneously represents the engine rotation stop position due to the reverse rotation.
Hall element type sensors may alternatively be used in place of the electromagnetic sensors. The Hall sensors cost more than the electromagnetic sensors.
Recent internal combustion engines have a variable valve timing (VVT) control mechanism, which varies a camshaft rotation angle relative to a crankshaft rotation angle thereby to vary the open/close timing of intake valves and exhaust valves relative to an engine rotation position. This mechanism uses an electric motor as a driving source for the camshaft as proposed in PCT publication WO 2004/038200 A1, which corresponds to U.S. patent application Ser. No. 10/510,765. To accurately control the camshaft rotation, the motor rotation must be controlled accurately in accordance with engine operating conditions. Therefore, a rotation position sensor is also used to detect a rotation position of the motor. A Hall element type sensor or other types of sensors are used as the motor rotation position sensor.